perf tools: Skip running the feature tests for 'make install-doc'

It is a requirement from the perf TODO list[1]:

''The feature tests should be performed only when a file that needs those
  tests, or at least only when some .c or .h file will be rebuilt. An
  initial step would be for 'make install-doc' not to run the feature
  tests, there it is not needed at all.''

By adding 'install-doc' to the NON_CONFIG_TARGETS, it will skip running
the feature tests for such target. The Auto-detecting system features
list will not be displayed:

  $ make install-doc
    BUILD:   Doing 'make -j2' parallel build
    SUBDIR   Documentation
  make[2]: Nothing to be done for 'install'.

[1] https://perf.wiki.kernel.org/index.php/TodoSigned-off-by: Rui Teng <rui.teng@linux.vnet.ibm.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1470818948-17784-1-git-send-email-rui.teng@linux.vnet.ibm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile.perf

@@ -165,7 +165,7 @@ SUBCMD_DIR	= $(srctree)/tools/lib/subcmd/
 # non-config cases
 config := 1
 
-NON_CONFIG_TARGETS := clean TAGS tags cscope help
+NON_CONFIG_TARGETS := clean TAGS tags cscope help install-doc
 
 ifdef MAKECMDGOALS
 ifeq ($(filter-out $(NON_CONFIG_TARGETS),$(MAKECMDGOALS)),)